Why don't all versions of an RPM show up in group repositories?

Update: On December 10 2015 RedHat released a version of createrepo for RHEL 7 which has a fix for this issue: "0.9 .9-24 - mergerepo: merge package versions from multiple repos. - Related: bug#1178763"

They also released a version for RHEL 6 shortly after that fixes the issue: "0.9.9-23 - mergerepo: merge package versions from multiple repos. - Resolves: bug#1175925"

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The "mergerepo" command in createrepo-0.10.1 and earlier eliminates duplicates at the artifact level. This has the effect of removing all but the latest version of an artifact in the generated metadata. This behavior has been fixed in newer versions of createrepo:

https://bugzilla.redhat.com/show_bug.cgi?id=1020756

As of this writing (December 9, 2014) neither CentOS/RHEL 6 or CentOS/RHEL 7 has the fix for this.